Modern skin care formulations must meet high standards of efficacy, skin compatibility and aesthetic appeal. Consumers are interested in mitigating or delaying the dermatological signs of chronologically-aged, hormonally-aged or photo-aged skin, such as fine lines, wrinkles, drying, and sagging skin, and other conditions due to a progressive degradation of the skin matrix. Consumers are interested in improving the appearance of, for example, skin, lips, nails, and hair by imparting to these biological surfaces a certain color, which would ideally produce an appearance of a uniform, lively, smooth and even surface, with no apparent imperfections. Therefore, there is a need for cosmetics that assist in creating a flawless, long lasting, lively coloring to improve the appearance of the biological surfaces.
Presently, make-up compositions that are applied to biological surfaces to impart a certain color such as foundations, face powders, eyeshadows, lipsticks, concealers, blushers, mascaras, eyeliners, lip pencils, eye pencils, or nail varnishes have difficulty achieving a perfect, flawless lively color because cosmetic ingredients such as coloring agents, which provide the desired color and coverage, generally have many limitations.
The coloring agents employed in such make-up compositions can be lakes, inorganic or organic pigments and/or pearlescent pigments, and alternatively dyes. Inorganic pigments, and in particular inorganic oxides, have the advantage of being relatively stable, but have the drawback of imparting rather dull, pale colors to the material being colored. Organic lakes have the advantage of imparting lively colors to the compositions, but are relatively unstable with respect to light, temperature or pH. Some of these coloring agents also have the drawback of leaving unsightly marks on the skin or the nails after application. Pearlescent pigments allow varied, but not intense, colors to be obtained with iridescent effects. Moreover, certain coloring agents have the drawback of generating free radicals in make-up formulations, which modify the color of the applied make-up and the stability of the compositions. Free radicals, when present on the skin promote ageing of the skin such as the appearance of wrinkles, fine lines and yellowing of the skin.
Therefore, there remains a need for cosmetic formulations and preparations that provide the increased color chroma of organic lakes while having the stability of inorganic pigments.
The compound eyes of insects are composed of ommatidia. The ommatidia have smooth surfaces, but some, such as those of moths and butterflies, are covered with tiny, slightly tapered protuberances. These structures are approximately 200 nanometers in both height and diameter at their base, and are arrayed across the surface of the ommatidia in a regular hexagonal pattern. These structures were first observed in nocturnal moths by W. H. Miller and colleagues in 1962 (Bernhard C. G. and Miller W. H. “A corneal nipple pattern in insect compound eyes,” Acia Physiol. Scand. 1962;56:385-386). Such structures are shown and described in Vukusic, et al., Nature 2003, 424:852-856, for example, FIG. 7.
Because the species that possess these structures tend to be active at night or in the dark, it is important that they absorb as much of the available light as possible. The function of such protuberances seems to be to reduce reflection of light from the surface of the ommatidia and thereby increase light's absorption by the receptor cells underneath. Like much of the exoskeleton of insects, the surface of each ommatidium is made of chitin, which has a refractive index (1.55) higher than that of air (1.00).
The protuberances work by providing a gradual transition in refractive index from air to ommatidium. Each individual photon that is incident on the ommatidia first encounters the thinner tops of the protuberances, making the effective refractive index only slightly higher than the refractive index of air. As the protuberance widens closer to the bottom, the refractive index of the surface approaches that of pure chitin. Because the size and periodicity of the protuberances are smaller than those of the optical wavelengths absorbed (<˜500 nm), each individual photon encounters this gradual transition, and reflection from the surface is minimized. This is known as the “moth-eye principle” or the “moth-eye effect”.
The moth-eye structure is well suited for many antireflective tasks. In the present application, the inventive cosmetic formulations permit increased light absorption and provide increased color chroma while being relatively stable.